1. Field of the Invention
The present invention is generally directed to a metal oxide varistor having a heat protection, especially to a metal oxide varistor integrally formed with an automatic switching-off structure that automatically goes to open circuit in conditions of overheating due to sustained over-voltages.
2. Description of the Prior Arts
Metal oxide varistors are widely used in circuits as voltage protection elements and inrush-current-absorbing elements. The metal oxide varistors have the capability of clamping high transient voltages appearing on unconditioned power lines to a low level for protecting electrical equipment or devices connected to the line. While the metal oxide varistors have a long life and have the ability to repeatedly clamp high transient voltage spikes to a safe level, the metal oxide varistors do eventually fail and ultimately, even if a catastrophic failure does not occur, the impedance of metal oxide varistors decreases to the point where they present a significant load, and eventually overheat and fail emitting smoke and fumes.
Thus, the protection is generally provided to a metal oxide varistors by connecting the varistors across the power line in series with a current limiting fuse and/or a thermal fuse. If the temperature of the varistor increases beyond the rated temperature of the thermal-fuse, the thermal fuse will open, thereby removing the varistor from the circuit. The thermal-fuses heretofore use to protect electrical circuits from varistor failure are generally cylindrical in shape, and have been mounted on the same printed circuit boards to which the varistor is mounted with the fuse arranged adjacent and parallel to the varistor body. As long as the thermal protective fuse is physically close enough to the varistor, an increase in varistor temperature will increase the temperature of the thermal protective fuse, causing it to open. While these thermal protective fuses heretofore used to protect electrical circuits from varistor failure have been somewhat effective, varistors may overheat in failure if localized overheating occurs at a portion of the varistor body remote from the fuse, the varistor and surrounding areas may be substantially destroyed before the temperature at the location of the fuse increases sufficiently to cause the fuse open. In a further known prior art device, in which a flat thermal fusible layer is deposited on a metal oxide varistor element. Another device provides protection by utilizing a spaced-apart lead from an electrode and is connected to it by a column of solder joints extending outwardly from the electrode. While these varistors appear to be reasonably effective, it still suffers from some drawbacks. It is doubtful whether a reliable insulation gap can be formed after fusing with relying on properties such as outgassing in an epoxy.
To overcome the shortcomings, the present invention provides a metal oxide varistor integrally formed with a heat protection structure for better handling of transient peak currents with desirable improvement of simplified manufacturing to mitigate or obviate the aforementioned problems.